WASHINGTON — President Obama declared clear, public restrictions for the first time on using unmanned aircraft to kill terrorists, a shift likely to significantly reduce U.S. drone strikes in Pakistan and elsewhere.

He also lifted a ban on sending scores of prisoners held at Guantánamo Bay, Cuba, back to their home countries and renewed his call to move the remaining detainees onto U.S. soil for imprisonment and possible trial in civilian or military courts.

The announcements came in a major speech Thursday in which Obama defended his largely covert efforts against al-Qaida and other extremist groups as legal and morally justified. At the same time, he sought to narrow and redefine the scope of the worldwide anti-terrorism campaign he inherited from his predecessor, George W. Bush.

“Our systematic effort to dismantle terrorist organizations must continue,” Obama said. “But this war, like all wars, must end.

“We must define our effort not as a boundless ‘global war on terror’ but rather as a series of persistent, targeted efforts to dismantle specific networks of violent extremists that threaten America.”

The speech set a new road map for U.S. policy and a clear pivot for Obama, who had substantially expanded targeted killings abroad. His administration had refused for years to even acknowledge the classified drone operations conducted by the CIA and the military. But Congress and international allies have increasingly challenged the drone policy and the secret legal framework behind it.

Saying the effort is now at a “crossroads,” Obama indicated the drone campaign that has killed several thousand suspected terrorists and militants, and an undetermined number of civilians, will now decline sharply.

The need for strikes has receded, he said, because the threat from a weakened al-Qaida leadership in Pakistan has diminished, and U.S. troops will withdraw from neighboring Afghanistan over the next 19 months.

Obama also said he would consider proposals for an independent panel, or a special court, to review evidence before a drone strike is authorized.

Near the end of Obama’s address at the National Defense University in Washington, D.C., he was interrupted three times by a co-founder of the anti-war group Code Pink. Medea Benjamin implored him close the Guantánamo Bay prison. In three separate episodes, the president traded words with Benjamin, asking her to let him finish.

“Why don’t you sit down, and I will tell you exactly what I’m going to do,” Obama said, ending their first exchange. “Thank you, ma’am.”

But soon, she was back, apparently not happy with Obama’s comments about sending detainees to other countries and pleading with him to “release them today.”

After two more exchanges with Benjamin, 60, Obama tried to lighten up the situation.

“I’m going off-script, as you might have expected,” he said with a laugh. “The voice of that woman is worth paying attention to. Obviously I do not agree with much of what she said, and obviously she wasn’t listening to me and much of what I said. But these are tough issues, and the suggestion that we can gloss over them is wrong.”

The speech came the day after Obama signed classified-policy guidance that sets new standards for deciding whom to kill, where, and under what circumstances. For the first time, aides said, the United States will use lethal force only against a target that poses a “continuing, imminent threat to U.S. persons,” not those who posed a “significant threat,” the previous standard.

Officials said that under the new policy, the military, rather than the CIA, will take the lead on most strikes. That’s expected to be particularly true in Yemen and Somalia.

The new guideline is likely to ultimately rule out so-called signature strikes against massed groups of men believed to be fighters but whose identities are unknown. Critics said those attacks caused numerous civilian casualties and were a prime source of friction with Pakistan, where the drone attacks are deeply unpopular.

Under the new standards, U.S. authorities also must ascertain with “near certainty” that civilians will not be injured or killed in a drone strike. Strikes against foreign militants will now be conducted under the same standard as those against U.S. citizens who have joined forces with al-Qaida, according to a senior administration official.

Throughout the speech, Obama offered a nuanced view of U.S. policy. He defended his decisions, particularly against critics on the left, even as he said it was time to move from a “perpetual wartime footing” to a less ambitious campaign.

“To do nothing in the face of terrorist networks would invite far more civilian casualties — not just in our cities at home and facilities abroad, but also in the very places … where terrorists seek a foothold,” he said.

“Let us remember that the terrorists we are after target civilians, and the death toll from their acts of terrorism against Muslims dwarfs any estimate of civilian casualties from drone strikes.”

Leading Republicans accused the president of underplaying the danger of terrorist groups. “The troubling reality is that the president continues to underestimate the serious threat that al-Qaida and its affiliated and inspired terrorists present to Americans,” said Rep. Ed Royce of California, chairman of the House Foreign Affairs Committee. “Now is not the time to abandon robust efforts to keep Americans safe.”

Civil-liberties advocates said Obama did not go far enough.

“The president still claims broad authority to carry out targeted killings far from any battlefield, and there is still insufficient transparency,” said Anthony Romero, executive director of the American Civil Liberties Union. “The time to take our country off the global warpath and fully restore the rule of law is now, not at some indeterminate future point.”

As expected, Obama disclosed moves to reduce the inmate rolls at Guantánamo Bay, where about 100 of the 166 prisoners have joined a hunger strike.

He lifted a self-imposed ban on sending prisoners cleared for release back to Yemen, although he said each case would be reviewed again.

The end of the Yemen restrictions is key, given that 30 of the 56 prisoners eligible for transfer are Yemeni. Obama halted all transfers to the poor Middle Eastern nation in 2010, after a man trained in Yemen was convicted in a failed bombing attempt of an airliner over Detroit.

In a statement from its embassy in Washington, the Yemeni government said it welcomed the administration’s decision and pledged to “work with the United States to take all necessary steps to ensure the safe return of its detainees.”

Obama also said he would appoint a new senior envoy at the State Department and the Pentagon to arrange additional transfers. He also said he had asked the Defense Department to designate a site on U.S. soil to hold military commissions for terrorism suspects.

Material from The Associated Press,

The New York Times and The Washington Post is included in this report.